The laser beam provides an economic and rapid method of writing, bar coding and decorative marking of plastics. This technique is advantageous over current printing technologies because of the ease at which the layout can be adjusted using graphic computer programs and integrated into a production line. Laser marking methods are processes that use laser light to place characters, such as letters, figures, and symbols, bar codes or images on a surface by irradiating the target surface with laser light. Laser marking is a contact-free procedure; so that marking is possible even on soft, irregular surfaces that are not readily accessible. Laser marking is ink-free, which provides long-lasting applications and it is solvent-free, which makes it friendlier to the environment.
There are several laser types available for marking plastic surfaces. A CO.sub.2 laser allows speeds up to 10,000 mm/sec. At 10,600 nm, the CO.sub.2 laser enables laser marking by thermochemical reaction, melting, vaporizing and engraving. An Nd:YAG laser allows speeds up to 2000 mm/sec. The Nd:YAG laser at 1064 nm provides laser marking by carbonization, sublimation, discoloration, foaming and engraving. At lower power levels at 532 nm, the Nd:YAG laser marks by leaching or selective bleaching of dyes and pigments. These lasers have good flexibility in text and imaging and broad flexibility in marking based on several phenomena, such as melting, foaming, vaporizing and engraving.
In one type of laser marking, it is desirable to obtain a light and/or colored contrast on a dark background. This type of laser marking can be accomplished by making white markings against a dark background or by using dyes or pigments to create colored markings, such as red, blue or yellow against a dark background. A problem that often occurs with contrasting laser markings is that the dark background pigment will interfere with and dull the brightness of the light pigments or the white laser marking, such as by leaving a grayish or yellowish tint. However, if a sufficient amount of the dark pigment is not used, the intensity of the dark background color decreases and the desired contrast between the light laser markings against the dark background is not achieved. Often, the light pigment colors distort the dark background color so that the color of the light pigment can be recognized in the dark background color.
Carbon black is often used to create a black background color. The irradiation from the laser beam decomposes the carbon black into volatile components. These volatile components as well as the absorption of heat from the laser beam foam the surface, which scatters the light and leaves a lightly colored impression. In order to retain the desired contrast between the light color and the dark background color, carbon black loadings should not exceed about 0.05% of the composition. However, at this level, when striving for colored markings, the dye or pigment distorts the black background color.
U.S. Pat. No. 4,769,310 to Gugger et al. describes a method of laser marking for ceramic materials using an inorganic pigment.
Some prior art laser marking patents use animal charcoal or elemental carbon to create the dark background color, such as U.S. Pat. No. 5,350,792 to Hess et al. (describes a plastic molding composition for laser marking based on an organic thermoplastic polymer containing a black mineral pigment, such as animal charcoal) and U.S. Pat. No. 5,599,869 to Kurz (describes a plastic molding composition for laser marking based on an organic thermoplastic polymer containing a black mineral pigment with elemental carbon and another coloring agent).
U.S. Pat. No. 5,373,039 to Sakai et al. describes a laser marking composition comprising a thermoplastic resin, a compound of tetrazole, azo, sulfonylhydrazide or nitroso, and carbon black.
U.S. Pat. No. 5,866,644 to Mercx, et al. discloses polyester laser marking compositions having light colored markings on a dark background comprising a hydrated metal phosphate and mica.